Circuits for simulating an electrical load are frequently used in cases where a circuit to be tested—referred to as test circuit—is to be checked for the functionality thereof, without having to bring the test circuit to its “real” working environment.
Typical application fields include simulators—test environments—, for example, for testing control units having power electronic interfaces. In this case, the control units—for example, motor controllers—make up the test circuit, the intention being to test whether the control units react in the desired manner, thus whether the control unit reacts to specific state variables—received via its interfaces—by outputting—via its interface—appropriate output quantities. To that end, the environment that is relevant to such a test circuit is completely or partially simulated. For example, in the case of a motor controller, the motor to be controlled can be—completely or partially—simulated, for example, with the aid of a simulation computer or a plurality of simulation computers having I/O interfaces. For this purpose, a mathematical map of the motor is first created, which brings the characteristic data and state variables of the motor into a calculable interrelationship. The variables—control signals—acting on the—simulated—motor are received by the simulation computer via an I/O interface, and state variables of the—simulated—motor are calculated in the simulation computer, inter alia, on the basis of this information using the mathematical model. Specific state variables are typically made available to the motor controller again via the I/O interface, respectively the I/O interfaces, to the extent that they are requested by the motor controller as inputs. This approach generally has the important inherent advantage that a large spectrum of test cases can be tested while entailing only little outlay, and modified control unit environments—for example, different drive units—can also be simulated.
In the motor controller example under consideration here, such a simulator not only receives signals in the low-level signal range from a test circuit, but also high-level electric signals, when the test circuit is provided with power electronic outputs, as is the case, for example, in the control of electric drives.
In practice, the circuits for simulating an electrical load are frequently operated in such a way that the voltage at the output of the test circuit—thus, for example, the voltage at the output of the power section of a motor controller—is measured metrologically; a corresponding (motor) current, that would have to flow over the terminal of the test circuit, is calculated using a mathematical model of the motor to be simulated, taking into consideration the operating data of the motor; and this setpoint current value is transmitted to the current-control unit which then adjusts the ascertained setpoint current as closely to real time as possible by controlling the circuit at the terminal of the test circuit accordingly.
The World Patent Application WO 2007/042228 A1 describes a circuit, for example, that uses a coil as an electrical energy storage device, whose inductance is substantially lower than that of the winding of an electric motor to be simulated. Controlling an electric motor typically requires a plurality of terminals because such drives, which are operated at relatively high power levels, are to be controlled in a multiphase—usually three-phase—operation. Present at the terminal of the test circuit is typically a pulse-width modulated (PWM) voltage signal, via whose pulse duty factor, the voltage present on average over time at the terminal can be adjusted. The coil is connected by its other terminal via a half-bridge circuit to two auxiliary voltage sources, so that, by switching the one switch of the half-bridge circuit, this second terminal of the coil can be connected to a high potential, and, by switching the other switch of the bridge circuit, the second terminal of the coil can be connected to a very low potential. It is thus possible to influence the current flow within the coil and to adjust, respectively control the actual value of the current at the terminal of the test circuit to the value of a predefined setpoint current.
Under the related art, the voltage source is composed of the supply voltage of the test circuit and of two auxiliary voltage sources connected thereto. Energy can be altogether withdrawn from the circuit by bringing one of the two auxiliary voltage sources onto load, thereby reducing the current in the coil.
Metal-oxide-semiconductor field-effect transistors (MOSFETs) are typically used as switches for the half-bridge circuit. They allow considerably high switching frequencies. Therefore, high switching frequencies are required for the MOSFETs because a “smooth” curve of the actual current can only be achieved by activating the half-bridge circuit at high frequency. For this reason, the switching frequency of the MOSFETs in the half-bridge circuit is much greater than the frequency of the PWM voltage signal at the output of the test circuit.
Very high currents and voltages are to be applied by the energy stage of the test circuit, for example, when drive units of passenger cars having an electric drive, as used, for example, in hybrid or electric-powered vehicles, are to be controlled. It is not unusual for drives of this kind to be operated at power levels within the range of some 10 kW up to approximately 100 kW. Particularly when working with very dynamic load variations, it is necessary to have control over voltages at the terminal of the test circuit that are within the kV range and over currents that may be within the range of some 10 A and, at peak, several 100 A. However, at such high power levels, which are to be converted in the simulated electrical load, the MOSFET circuit elements described above reach their limits since they are no longer able to block the voltages occurring during switchover operations, without causing destruction, due to a too low dielectric strength. It is not readily possible to use more rugged circuit elements, such as IGBT transistors (insulated gate bipolar transistors), for example, since the requisite high switching frequencies are not reached in this case.
Some of the related-art approaches have the further disadvantage that comparatively high power levels are converted in the circuit for simulating a load, entailing significant power losses.